Diabetes is a term that refers to a collection of diseases resulting in disordered energy metabolism and varying degrees of blood glucose elevations or hyperglycemia. One of the best characterized forms of the disease is that which arises from an immunologically mediated destruction of the insulin secreting pancreatic beta cells. This severe form of the disease is termed Insulin-dependent Diabetes (IDD or IMD) since it is associated with progressive insulin deficiency and coincident symptoms such as weight loss, glycosuria and polyuria, and increased thirst or polydipsia. Other terms for this form of diabetes are Type 1 Diabetes (cf. Type 2 Diabetes which results from an inherent resistance to insulin action); Ketosis Prone Diabetes because there is abnormal generation of ketone bodies as a result of excessive breakdown of body fats due to the severe insulin deficiency; or Juvenile Diabetes, since virtually all diabetes that appears in childhood and adolescence is of this type.
Diabetes is a major public health problem, especially in Western countries. The incidence rates vary greatly worldwide, from as high as 40 per 100,000 persons in Finland to as low as 1-2 per 100,000 among the Japanese. The peak incidence is during the pubertal years, associated with the increasing bodily demands for insulin associated with muscle growth. The prevalence rates in the United States population under age 20 years is 0.25% and it approaches 0.4% over a lifetime, albeit an estimated 10-20% of patients with Non Insulin-dependent Diabetes (NIDD) or Type 2 or Maturity Onset Diabetes also have, in reality, slowly progressive IDD. Thus, it is estimated that there may be at least 1 million Americans affected by IDD.
Diabetes results in progressive damage to the blood vessels of the body, to a degree that depends upon the severity of hyperglycemia and its duration. The incident mortality rate for IDD has been calculated to be 7-fold higher than for age matched non-diabetic controls. Whereas the decade long Diabetes Control and Complications Trial (DCCT)—concluded in 1994 by the National Institutes of Health in the United States—showed that meticulous insulin replacement therapy would slow the appearance of damaged arteries, it was not able to completely prevent this damage since blood glucose levels were difficult to keep within normal limits. Ocular complications of diabetes are the leading cause of new blindness in persons 20-74 years of age. The risk of lower extremity amputation is 15-fold higher in those with diabetes. Approximately 40% of persons undergoing renal transplantations have kidney failure because of diabetes, and the proportion due to diabetes continues to rise each year. Women with diabetes produce newborn infants with a 7% newborn mortality rate. Other complications of diabetes include increased heart disease and stroke, loss of nerve cells or neurons enervating the limbs and intestine, impotence and infertility, cataract formation in the lens of the eyes, increased periodontal disease, and predisposition to infectious diseases especially from bacteria and yeast. Of all patients with diabetes, those with IDD have a disproportionate share of these complications because of its severity and usual early age of onset. In the United States, the direct health care costs attributable to diabetes in 1994 have been estimated to exceed $120 billion. Thus it is important that the pathogenesis of IDD be understood and strategies be developed to prevent it as a fully expressed clinical disease.
Patients with IDD are unusually prone to other diseases that have become recognized as having autoimmune origins. These diseases include thyroiditis or Hashimoto's disease, Graves' disease, Addison's disease, atrophic gastritis and pernicious anemia, celiac disease, and vitiligo (Maclaren, N. K. [1985] Diabetes Care 8(suppl.):34-38). Evidence that IDD itself has an autoimmune nature began with histological studies of patients; these studies indicated that the islets were infiltrated with a chronic inflammatory (lymphocytic) infiltrate termed insulitis. This was supported in the early 1970s by reports of islet cell autoantibodies reactive to antigens within the cytoplasm (ICA) (Lendrum et al. [1975] Lancet 1:880-882) or confined to the islet cell surfaces (ICSA) (Maclaren et al. [1975] Lancet 1:977-1000) as detectable by indirect immunofluorescence. Later it was recognized that many patients also develop autoantibodies to insulin (IAA) before their diagnosis (Palmer et al. [1983] Science 222:1337-1339) as well as to insulin receptors (Maron et al. [1983] Nature 303:817-818). Autoantibodies were also reported to an islet cell protein composition of 64,000 M.Wt. in man (Baekkeskov et al. [1982] Nature 298:167-169), in the Biobreeding (BB) rat model (Baekkeskov et al. [1984] Science 224:1348-1350), and in the Non Obese Diabetic (NOD) mouse model (Atkinson and Maclaren [1988] Diabetes 37:1587-1590). 64 kDa antigen has subsequently been reported to be the lower molecular weight isoform of glutamic acid decarboxylase (GAD65) (Baekkeskov et al. [1990] Nature 347:151-156) (Kauffman et al. [1992] J. Clin. Invest. 283-292). GAD is an enzyme that converts glutamate into the membrane stabilizing neurotransmitter called gamma amino butyric acid or GABA. In addition to autoantibodies to GAD, peripheral blood mononuclear cells were shown to be autoreactive in patients developing IDD (Atkinson and Maclaren et al. [1992] Lancet 339:458-459; and Harrison et al. [1993] Lancet 341:1365-1369).
It has previously been demonstrated in several autoimmune diseases, including IDD, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), multiple sclerosis (MS), and autoimmune thyroid disease, that antigen-presenting cells (APCs) such as monocytes and macrophages are dysfunctional in their ability to activate T lymphocytes (Via, C. S. et al. [1993] J. Immunol. 151:3914-3922; Serreze, D. [1993] FASEB J. 7:1092-1096; Rasanen, L. et al. [1988] Clin. Exp. Immunol. 71:470-474; Hafler, D. A., et al. [1985] J. Neuroimmunol. 9:339-347). The defect(s) in APC function, however, have thus far not been defined at the cellular or molecular level.
Prostaglandins (PGs) are lipid molecules formed from a precursor molecule, arachidonic acid, through the actions of specific enzymes called prostaglandin synthases (PGS-1 and PGS-2). PGS-1 mRNA and protein are constitutively expressed, and this enzyme is responsible for the production of low levels of PGs and functions as a housekeeping molecule. PGS-2 is an inducible enzyme expressed by macrophages and monocytes during inflammation and following exposure to mitogens, cytokines, and bacterial cell wall products, i.e., lipopolysaccharide (LPS) (Farber, J. M. [1992] Mol. Cell. Biol. 12:1535-1545; Vane, J. R. [1994] Proc. Natl. Acad. Sci. USA 91:2046-2050; Kujubn, D. A. [1993] J. Biol. Chem. 266:12866-12872; Ristimaki, A. et al. [1994] J. Biol. Chem. 269:11769-11775). PGS-2 has been shown to be expressed in the cells lining the joints of individuals with rheumatoid arthritis and may contribute to the ongoing inflammation in the affected joint (Crofford, L. J. et al. [1994] J. Clin. Invest. 93:1095-1101).
In addition to prostanoids, monocytes produce monokines that strongly modify inflammation and immune responses. Among the monokines that in general upregulate or promote inflammation and immunity are TNFα, IL-1α, IL-1β, IL-12, and IL-6. Monokines that tend to downregulate these responses are IL-4, IL-10, IL-13, and IL-1 receptor antagonist (IL-1RA).
Prostaglandins are known to have an effect on the expression of monokines. For instance, prostaglandin E2 (PGE2) is known to suppress the production of TNFα (Seldon, P. M. et al. [1995] Mol. Pharmacol. 48:747-757; Strieter, R. M. et al. [1990] J. Leuk. Biol. 47:366-370). There are also reports that PGE2 suppresses IL-1α (Endres, S. et al. [1991] Immunology 72:56-60; Zhong, W. W. [1995] Immunology 84:446-452). In contrast to its suppressive effects, PGE2 production stimulates the production of IL-10 by monocytes (Strassman, G. et al. [1994] J. Exp. Med. 180:2365-2370). IL-10 in turn modulates PGE2 production by suppressing PGS-2 production (Mertz, P. M. et al. [1994] J. Biol. Chem. 269:21322-21329). In addition, IL-10 stimulates the production of another potent immunoregulatory monokine, IL-1RA (Spengler, R. N. et al. [1989] J. Immunol. 142:4346-4350). PGE2 therefore can stimulate immunoregulation through its own action and through its actions on monokine production.
If monocytes are chronically exposed to PGE2 in vitro, there is a loss of response to its action. This desensitization phenomena is mediated by the down-regulation of PGE2 receptors (Coffey, R. G. et al. [1990] J. Leuk. Biol. 48:557-564). For example, PGE2 in vitro normally suppresses TNFα production, but chronic exposure leads to a loss of suppression of this monokine by PGE2. Removal of, or blocking, PGE2 can reverse the desensitization process (Howard, M. et al. [1992] J. Clin. Immunol. 12:61-784).
Currently, one can screen for individuals at high risk for the development of IDD by serologic methods only, which reflect autoimmune B and T lymphocyte activity. Serologic tests identify approximately 80-85% of individuals who have existing autoimmune disease against the insulin-producing cells of the pancreas. Of the ICA+ population, approximately 80-85% will develop IDD within the ensuing five years. Currently no test exists, immunologic, genetic, or otherwise, which can identify individuals at risk for IDD other than these serologic tests.
Because IDD takes several years to develop in an individual, autoimmunity may be firmly established at the time that individuals develop ICAs. Detection of background cellular or genetic factors necessary for the development of autoimmune disease and expressed early in the disease process is of great clinical importance. Detection of these factors would preferably identify individuals before autoimmunity is initiated, or perhaps at earlier stages of the disease than detected by ICA. Earlier detection would be of great clinical importance in identifying individuals at high risk for disease where the administration of preventative therapies that attempt to preserve the residual insulin-secreting cells are employed.